I don't think scalability or reliability per se will be an issue for today's Postgres on modern hardware.
BUT - you're proposing building what is quite likely the busiest and most popular Postgres-backed site in the world. You'll be an early achiever, in other words. I can't point you to any data from existing OpenACS sites to support the notion that Postgres will give you the 24/7 reliability you need. Of course, I also can't point you to any data suggesting that it won't. We just don't have the data.
Now, I want to see this data exist, i.e. busy PG-backed OpenACS sites running out there so we can get a handle on the combination's scalability and reliability, but I can't honestly tell you that you should be the first major Postgres-backed site on the block. That's something you'll have to decide.
If the others in your business share your desire to support Open Source, it's probably a reasonable path to take. If the others are conservative, could care less about Open Source, and want to take the lowest risk path, then Oracle's probably the right choice for you. It sounds like money's not an issue...
To put things in perspective, the use of Oracle doesn't guarantee a trouble-free operation, either. I'm working on a client site using ACS Classic (ACS+Oracle), and Oracle hosed itself last week. Crashed the database engine on a simple on-line "update", first from Tcl scripts then from SQL*Plus. Each and every time. We brought everything down, even rebooted Linux, and the crash remained. Had to drop tables and restore from a dump, evil evil. I've not had to do that yet with my Postgres-backed site that's been up for about a year.
